The present invention relates to a method of continuous stripping of an aqueous latex of a vinyl chloride resin suitable for paste technique, and more particularly to a method for continuously stripping unreacted monomer or monomers under reduced pressure from the latex of vinyl chloride paste resin, which is easy to foam during the stripping, while defoaming the latex without deteriorating the product quality and without lowering the productivity. The present invention also relates to an apparatus suitable for the removal of unreacted monomer or monomers by continuous vacuum stripping, and a vinyl chloride paste resin latex having a low concentration of residual vinyl chloride monomer.
In general, in the production of vinyl chloride paste resins, firstly a resin latex is obtained by micro-suspension polymerization, emulsion polymerization or seed-emulsion polymerization of vinyl chloride alone or a mixture of vinyl chloride and other vinyl monomers copolymerizable therewith. For obtaining the paste resin products, the thus obtained latex is then usually spray-dried. The obtained vinyl chloride paste resin powder is dispersed in a liquid plasticizer to form a plastisol, and the plastisol is used in the paste technique.
The term "latex of vinyl chloride paste resin" or "vinyl chloride paste resin latex" as used herein means an aqueous dispersion of a vinyl chloride homopolymer or copolymer usually having a particle size of 0.1 to 10 .mu.m prepared for the purpose of the paste technique. The latex may contain a surfactant such as an anionic surfactant or a non-ionic surfactant, as occasion demands.
In general, the vinyl chloride paste resin latex contains a large amount of unreacted monomer such as vinyl chloride. Therefore, prior to spray-drying the latex to obtain the paste resin powder, it is required to recover the unreacted monomer in a high efficiency, ideally a whole amount of the unreacted monomer, from the latex in order to reuse the monomer and in order to suppress the release of monomer into the atmosphere from the environmental point of view.
As a method for stripping an unreacted monomer from an aqueous liquid containing a vinyl chloride resin are known, for instance, a batchwise method wherein the aqueous liquid is transferred into a pressure container such as tank and the unreacted monomer is recovered under reduced pressure with heating, a method as disclosed in Japanese Patent Publication Kokoku No. 53-38187 wherein steam is blown into an aqueous polymerization reaction mixture under reduced pressure, a continuous stripping method as disclosed in Japanese Patent Publication Kokai No. 54-8693 wherein the aqueous liquid is introduced into a perforated plate column and is brought into counter current contact with steam introduced from the bottom of the column. However, if these methods are applied to latex of a vinyl chloride paste resin, a problem of foaming of the latex is encountered. That is to say, the latex contains a large amount of a surfactant in order to retain the stability, whereby vigorous foaming of the latex occurs to fill the container or column with foam and the foam is scattered into a monomer recovery pipe. The scattered foam may cause failure or trouble of equipment. Also, the recovery pipe may be filled with the foam to increase the pressure loss in the pipe, thus resulting in lowering of the recovery ability of an exhaust pump.
In order to prevent the foam generated during the monomer recovery under reduced pressure from entering into the exhaust pipe for the monomer recovery, a suitable defoaming agent can be added to a vinyl chloride paste resin latex or the flow rate of the monomer recovered can be decreased. However, the addition of defoaming agent into the latex may introduce a problem of deterioration in paste resin quality, since a large amount of the defoaming agent is required in the defoaming of such a latex and may change the physical and chemical properties of vinyl chloride paste resins, although the quality is not adversely affected by the addition of a small amount, e.g., several hundreds ppm, of defoaming agent. The decreasing the flow rate of the recovered monomer requires a long time for recovering the monomer gas, thus resulting in lowering of the productivity.
Also, it is proposed to install a foam separator in an exhaust line for the recovery of unreacted monomer from a vacuum tank, whereby the foam scattered from the vacuum tank into the exhaust line is stored to prevent the foam from entering into an exhaust recovery pipe connected to the separator. However, since the vinyl chloride paste resin latex has strong foamability and foam retainability, the foam separator is filled with foam in a short period of time and the foam may enter the recovery pipe.
Another defoaming method is also proposed in Japanese Patent Publication Kokai No. 2000-212214 wherein a foam generated in a vacuum recovery tank during vacuum stripping is defoamed by bringing the foam into contact with a cooling means provided in the space of the vacuum recovery tank or in an exhaust pipe connected thereto. This method is effective for defoaming the vinyl chloride paste resin latex. However, in case that the vacuum stripping is carried out by introducing steam to the recovery tank, there arises a problem that steam is condensed at the cooling portion to lower the solid concentration of the latex and, therefore, the productivity in spray drying of the latex in the subsequent step is lowered.
Like this, known stripping methods are not satisfactory from the viewpoint of defoaming of vinyl chloride paste resin latex or productivity. Satisfactory stripping method and apparatus suitable for vinyl chloride paste resin latexes have not been proposed.
It is an object of the present invention to provide a continuous method for stripping unreacted monomer or monomers such as vinyl chloride under reduced pressure from a vinyl chloride paste resin latex, which is a polymerization reaction mixture and is easy to foam during the stripping due to the presence of a surfactant, while defoaming without deteriorating the quality of the vinyl chloride paste resin and without lowering the productivity.
Another object of the present invention is to provide a vacuum stripping apparatus capable of recovering unreacted monomer or monomers in a high efficiency by a continuous operation from a vinyl chloride paste resin latex obtained by polymerization without causing any trouble resulting from foaming of the latex during the stripping.
A still another object of the present invention is to provide an aqueous latex of a vinyl chloride paste resin having a low concentration of residual monomer or monomers.
These and other objects of the present invention will become apparent from the description hereinafter.